Drain valves are, for example, used in refrigeration systems having a defrost function. When an evaporator of a refrigeration system works below 0° C. for a prolonged period of time water vapor from the ambient air can condense on the evaporator and form an ice layer on the evaporator. Ice is a relatively good heat insulator and therefore if the ice layer is allowed to build up it will reduce the efficiency of the refrigeration system more and more over time.
Consequently, a system for defrosting the evaporator is provided in many refrigeration systems. A typical way to defrost the evaporators is to divert hot gaseous refrigerant normally intended for the condenser into the evaporator to increase the temperature and remove any ice that may have accumulated on the evaporator. At the same time most of the liquid refrigerant in the evaporator is also pushed out by the hot gas. Since a mixture of liquid and gaseous refrigerant then exits the evaporator some of the hot gas may be fed back into the low pressure side of the circuit whereby the efficiency of the refrigeration system during a defrosting operation is reduced. In order to avoid such a reduction in efficiency it is known to provide a drain valve of the kind mentioned at the outset to ensure that the amount of hot gas that is fed back into the circuit is lowered. The drain valve prevents the passing of hot gas into the low pressure side of the refrigeration system during the defrost cycle. The flow rate of the liquid refrigerant is controlled in such a drain valve by a float connected to a lever which controls the opening and closing of a liquid outlet orifice through which the liquid refrigerant can exit the drain valve. When the level of liquid inside a float chamber of the drain valve rises the float connected to the lever also rises due to its buoyancy whereby the closing member opens the liquid outlet orifice and allows a larger amount of liquid to exit the drain valve. This allows to control the flow speed of the refrigerant coming from the evaporator back into the low pressure side of the refrigeration system.
However, in order to produce a sufficiently large force to displace the closing element and open the liquid outlet orifice the float and the float chamber need to have a relatively large size. In particular, opening the liquid outlet orifice will create a negative pressure close to the liquid outlet orifice as compared to the rest of the float chamber. This results in a net pressure force on the closing element that counteracts the force provided by the float. Consequently, if the pressure of the refrigerant in the drain valve is relatively high compared to the low pressure side of the refrigeration system and these pressure forces can become comparable to the force provided by the float. This consequently limits the functionality of the drain valve. The known solution to this problem is to increase the size of the float and the float chamber as well as the length of the lever to ensure that the drain valve will function properly even in higher pressure applications. However, this solution is undesirable since it increases both the overall size and costs of the drain valve.